In the specific assay methods based on bioaffinity the analytes are usually measured at very small concentrations, which require the use of labelling agents that are detectable by a very sensitive method. Such bioaffinity assays include inter alia immunochemical assays, nucleic acid hybridizations, lectin reactions as well as receptor assays. Various labelling agent methods are usually used in the analytical applications of all these reactions. The radioisotopes are conventional labelling agents used for example in radio immunological (RIA) and immonoradiometric (IRMA) assays, which are the most sensitive specific analytical methods used in the practice. The detection sensitivity of the RIA assays is ca. 10.sup.-14 M and the corresponding sensitivity limit of the IRMA assays is ca. 10.sup.-16 M. Despite the common usage, the radioisotopes as labelling agents present some drawbacks such as a limited lifetime as well as handling problems. For this reason, active research has been directed to possibilities to replace the radio active labelling agents with other alternatives.
The fluorescence methods are more and more widely used in chemical, biochemical and medicinal analytics. Fluoroimmunological and immunofluorometric assays that are based on time-resolved fluorometrics and on lanthanide chelates as labelling agents give at least the same or even better sensitivity compared with RIA and IRMA assays.